Analysis of metals and acids in Electropolishing Baths for stainless steel

A. The presence of hexavalent chromium ion will cause some inaccuracy in titration analysis. In our experience, keeping metal content below 3% via frequent, small decants maintains acid balance, and the inaccuracy of the titration is insignificant. The titration for metal content is usually satisfactory if the bath is not operated as a "full-sludging bath".

A. After diluting your sample, you can titrate it with NaOH and watch for two end-points: the first, v1 - when both protons of sulfuric acid and the first proton of phosphoric acid have been neutralized (around pH 4.5); and the second, v2 - when the second proton of phosphoric acid has been neutralized (around pH 9.5). You can then calculate the molarity of phosphoric acid from v2 - v1 and the molarity of sulfuric acid from [v1 - (v2 - v1)]/2 = v1 - v2/2 after accounting for proportionality through the known volumes and NaOH concentration.

The end-points are well separated and can best be detected potentiometrically or thermometrically. If you need to rely on indicators (e.g. bromocresol green ⇦ this on eBay & Amazon [affil links] for the first end-point and thymolphthalein for the second), you may have to titrate two samples, one for each end-point (unless somebody can think of a combination of indicators that works without mutual interference at these particular pH values).

A. In order to titrate iron in the solution you need to reduce it quantitatively to Fe(2+). A simple way to do that is to pass the sulfuric acid solution through a bismuth column. This is simply a small liquid chromatography column filled with 25-50 (or better, 50-100) mesh high purity Bi particles. I have used a 0.8 or 1.0 cm I.D., 20-25 cm long filled column. My experience has been with dilute (1-2 M) sulfuric acid. I don't think the phosphoric acid would interfere, at least thermodynamically speaking. If you decide to use this method, be sure to keep all the bismuth submerged in acid at all times, to rinse it well with acid before the analysis, to pass the sample solution through it at no more than about 20 ml/min (use a little vacuum in the receiving flask if gravity is not enough), and to rinse the column after the sample into the receiving flask with an amount of dilute sulfuric acid equal to at least 3 times its estimated intergranular empty volume. The solution in the receiving flask is immediately titrated with calibrated KMnO4 solution (watch for stable faint pink).

This technique is similar to the better known Jones (Zn/Hg) and silver reductors, but the column is much easier to prepare (pour Bi particles in; rinse with acid) and to maintain, and selectivity is quite high. I know that Ni, Co, Ti and Sn(4+) do not interfere, and I am sure the same is true of a few additional metals. If Cr is present, a K2Cr2O7 (dichromate) titration may have to be used instead of permanganate because permanganate can oxidize Cr(3+). Unlike permanganate, dichromate requires an end-point indicator or a potentiometric method. For a typical (not necessarily optimal) dichromate procedure see Volumetric Analysis of Metal Finishing Solutions [on Amazon affil links] by Andrew McFadyen, Finishing Publications Ltd.--, p. 154.

A. The ratio of sulfuric acid to phosphoric acid is not particularly critical, but many electropolishing solutions have other additives in them. I therefore suggest you check the details in the process sheet supplied by your solution's supplier. This will tell you all the analyses you will need to do and what the acceptable ranges will be. Successful electropolishing is dependent on how much metal is dissolved in the solution, its viscosity and operating temperature - detail of these parameters will also be in the process sheet.

A. Hi cousin Tri. Unfortunately, there are about a dozen individual titration calculations on this page for a number of different acids and metals in electropolishing solutions, ranging over 24 years, with some of the participants having passed away over the decades :-(

I appreciate that English may be your second language and that is causing you to try to be brief, but please introduce who you are, what you do, what individual material you want to analyze for in what kind of processing solution. Then hopefully someone can walk you through it. Thanks!

If you are totally lost rather than stuck on a specific step, googling "what is titration" or youTubing "titration" will help.

Luck & Regards,